System for electrically transmitting and reproducing sound



Dec. 16, 1930. J L, REYNOLDS 1,785,048

SYSTEM FOR ELECTRICALLY TRANSMITTING AND REPRODUGING SOUND Filed May 19. 1927 2 Sheets-Sheet l y AI TRANSMITTER w VOLUME ATTORNEY Dec. 16, 1930. J. L. REYNOLDS 1,785,048

SYSTEM FOR ELECTRICALLY TRANSMITTING AND REPRODUCING SOUND Filed May 19. 1927 2 sh t s 2 Illlllrr RECEIVER k I V I I I I H I I I =5 IIIIUII INVENTOR 0 a ATTORNEY TRANSMITTER Patented Dec. 16, 1930 UNITED STATES PATENT OFFICE I JOHN LOUIS REYNOLDS, OF LONG ISLAND CITY, NEW YORK, ASSIGNOR, IBY MESNE AS- SIGNMENTS, TO ELECTRICAL RESEARCH PRODUCTS INC., A CORPORATION 01' DELA- WARE SYSTEM FOR ELECTRICALLY TRANSMITTING AND REPRODUCING SOUND Application filed May 19,

This invention relates to amplifying systerns and in particular to systems of the type using electron discharge amplifying apparatus.

The principal object of the invention is to produce a sound amplifying system for amplifying sounds to any desired level in such a manner as to give, in a reproducer, the same effects as the original sounds.

It is known that the aural response of the human ear for any given frequency of sound waves impinging thereon, is a function of the amplitude or energy level of the waves, and that for a. given energy level of the arriving Waves the response is a function of the fre-' quency. It has been found in artificial sound transmitting and reproducing systems using mechanical devices for reproducing the sound that the eflect so far as the ear is concerned varies with the total volume of sound from the reproducing device and produces what may be termed a selective effect upon the ear. That is, for very low volume reproduction the vibrations in the lower end of' frequencies, whereas for very loud volumes the low frequency vibrations appear to be reproduced more efficiently than the higher frequencies and in some cases tend to mask the high frequencies. Thus the general tone of the reproduction may be varied by simply varying the total or average energy level of the reproduced waves. It is an object of the resent invention to produce a sound ampliying system suitable for use in artificial sound reproduction wherein the eificiency of amplification so far as the effect on the human ear is concerned, is substantially independent of the volume of sound desiredfor reproduction.

A feature of the invention relates to an amplifier of the electron discharge type hav ing a substantially fiat over-all gain-frequency characteristic.

Another feature has reference to an electron discharge amplifier comprising a series of stages with a frequency selective coupling between stages, and a volume control or gain control capable of being adjusted in unison 1927. Serial No. 182,616.

with the selective coupling to compensate for volume-frequency distortion.

A still further feature of the invention relates to an electron discharge am lifier suitable for use in amplifying soun modulated currents in an artificial sound reproducing system, whereby the faithfulness of the reproduced sound is independent of the general level of reproduction volume desired.

Other features and advantages of the invention not specifically enumerated above will reveal themselves. as a 'result of a consideration of the following detailed description, which taken in conjunction with the drawing, shows a preferred manner of carrying out the invention.

Referring to the drawing, Fig. 1 shows an explanatory si'mplified'diagram of the principles upon which the invention is based.

Fig. 2 shows curves illustrating the manner in which the impedance of the receiving circuit of Fig. 1 varies with frequency.

Fig. 3 illustrates an arrangement for amplifying the frequency components of a complex sound with equal efficiencies and independently of the degree of amplification.

Fig. 4 illustrates a series of curves representative of the manner in which the transmission of the several frequency components of a complex sound wave is dependent upon the volume of reproduction desired or in accordance with the gain of the amplifying apparatus. This figure also shows curves illustrating the manner in which the compensating relations vary according to volume of reproduction.

The numeral 1 (Fig. 1) represents a microphone, or similar device for varying the amplitudes of current from a suita le source in accordance with sound vibrations. These varying currents are impressed, by means of transformer 2, upon the reproducing device 3 in series with the variable condenser 4. It will be apparent that as the condenser is varied the impedance of the reproducer circuit and condenser also varies in the manner shown by the curves of Fig. 2 wherein the abscissm represent frequencies of the imressed currents and the ordinates represent impedances. Curve represents the conchtion with condenser 4 adjusted to maximum capacity. Curve B corresponds to the mean capacity and curve C to the minimum capacity. For a given capacity of condenser less impedance is offered to the higher frequency components of the sound currents and the lower frequency components are transmitted to the reproducer 3 with corresponding decreased amplitudes. For a large capacity, for example, that represented by curve A of Fig. 2, the lower frequency components are transmitted more readily. This increase in the transmission capabilities of the circuit due to the increase of capacity has the effect not only of allowing the lower frequencies to be reproduced but necessarily causes an increase in the total volume of sound from the reproducer. From an inspection of Fig. 2 it is obvious that the rate of change of impedance with frequency is dependent upon the value of the capacity 4 in circuit with the reproducer 3. For example, considering curve A, the rate of change of impedance is very slight, while for curve C the rate is materially greater. It will also be observed that for frequencies above 500 cycles per second the change in impedance is substantially independent of changes in the value of capacity from minimum to maximum, while for lower frequencies, the change for various values of capacity is very marked. For example, at 100 cycles per second the impedance with minimum capacity in circuit is more than ten times the impedance offered with the maximum capacity. It will be understood, of course, that this ratio between maximum and minimum impedances for a given frequency is dependent upon the electrical characteristics of the remaining portions of the circuit including the transformer 2 and the reproducer 3, but as is well understood in the art, these values may be proportioned to give characteristics substantially similar to those of Fig. 2.

As mentioned hereinabove, the human ear is not equally responsive to rapidly recurring sounds of different frequencies unless the energy of the respective sounds is substantially the same; that is to say, when the ear is required to respond to sounds of great intensity the low frequency components of these I sounds appear to excite a greater response per unit of energy than the higher frequency components, and as the ear is the final arbiter as to the faithfulness of reproduction this characteristic causes a complicated sound wave to' produce different effects on the ear according to the intensity of the sound. This variation in the aural effects of a reproduced sound, while it may not be noticeable under ordinary conditions employing very low volume reproduction, becomes marked when the sound 1s to be reproduced, for example, in a large auditorium where the maximum attainable volume without distortion is required.

In order to compensate for this volumefrequency distortion effect, according to the invention an amplifier is provided that amplifies the higher frequency components with a greater gain than the lower frequency components, according to the degree of volume required for reproduction.

Referring to Fig. 3, the rectangle 5 represents a source of sound-modulated currents which are impressed upon the grid and filament of the electron discharge amplifying tube 8, by means of the windings 6 and 7 of the input transformer T. A suitable source of polarizing potential 9 is also connected to the grid of tube 8 for enabling said tube to be operated on the linear portion of its characteristic curve. The filament of tube 8 receives heating current from the low voltage line A, it beingunderstood that appropriate means are provided for regulating the voltage appliedto the terminals of the tube filament. The plate of tube 8 is connected through impedance coil 10 to conductor 11 Which leads to the positive terminal of a suitable source for supplying the necessary plate potential, as is well known in the art. The current variations impressed upon the filament-grid input circuit of tube 8 are amplified and repeated into the plate or output circuit of this tube and cause corresponding varying potential differences at the terminals of the impedance coil 10. These latter variations in potential cause'corresponding currents to flow in the circuit, including the condenser 12, potentiometer resistance 13, polarizing battery 14:, and the negative terminal of the filament of tube 15. The amplified variations from tube 8 are thus translated into corresponding potential drops across the resistance 13. The grid of tube 15 is connected to the adjustable brush 16, whereby the gain of amplifying tube 15 may be varied in the Well-known manner. It will be understood that the coupling condenser 12 is of such a capacity as to transmit substantially all the component frequencies of the amplified modulated currents without undesirable attenuation. In a similar manner to that already described, in connection with tube 8, tube 15 amplifies and repeats the variations of its grid'potential into variations in potential across the impedance coil 17. These latter variations result in corresponding variations in potential across the section 18 of the auto-transformer T these potentials being stepped up by the section 19 and applied to the grid of the tube 20. The autotransformer T should preferably be of very high impedance so that the combination of its inductance and the capacity 22 does not provide a resonant circuit over the working range of variations of said condenser. Tube 20 amplifies and repeats the variations impressed upon its input circuit into the primary winding of the output transformer T the secondary winding-of which is connected to a suitable electromechanical reproducer schematically represented by the rectangle 21.

It will be noted that the condenser 22 for coupling the output of tube 15 to the input of tube 20 is variable, the purpose of which will be apparent after the description given hereinbelow. Referrin to Fig. 4, there are shown three curves a, c, which represent three different conditions under which the amplifier reproducer system of Fig. 3 may be operated, with a fixed setting of condenser 22. Curve a represents the manner in which the different frequency components of the reproduced sounds are perceived by the ear when the amplifier is adjusted to its minimum working gain. This curve is substantially flat, indicating that the low frequencies are heard equally as well as the higher frequencies. Curve 6 illustrates similar relations when the amplifier is adjusted to medium gain. Inspecting the curve 6, it will be seen that the sounds are perceived with an apparent falling off in volume as the fre quencies become higher and higher. Curve 0 represents the conditions when the amplifier is adjusted to maximum gain. In this curve it is seen that distortion of the higher frequencies as heard is still greater. In other words, with a given capacity coupling between tubes 15 and 20, the faithfulness of transmission and perception decreases as the volume increases. This may be designated as volume-frequency distortion.

One method of overcoming this distortion is to amplify the higher fre uencies to a greater degree than the lower requencies as the volume of reproduction is raised, in order to produce at the different volume levels substantially flat apparent characteristics. The combination of the variable condenser 22 and the amplifier tube 20 of Fig. 3 may be adjusti quency characteristic is represented by curve ed to give this compensation. For example, the condenser 22 may be of such a value that when it is adjusted to maximum capacity and the gain control potentiometer 13 is set to its minlmum working value, the volume-frea of Fig. 4. If it is desired to eti'ect the reproduction at a higher volume, for example, the medium volume, the gain control potentiometer is adjusted to its mean position and at the same time condenser 22 is adjusted to decrease the coupling between tubes 15 and 20, so that the actual frequency-gain characteristic of amplifier tube 20 has the form represented by curve I) of Fig. 4. Accordingly, it is obvious that the apparent characteristic is a curve such as the curve I)" which is substantially fiat. Similarly, ifit is desired to effect the reproduction at a very loud volume, the gain control potentiometer 13 is set to maximum setting and the condenser 22 is adjusted to give an over-all frequency-gain characteristic represented by curve a which results in the different frequency components being heard It is clear from the foregoing that the gain control potentiometer may be provided with a dial and graduated scale which has been previously calibrated to indicate not only the setting of the potentiometer for the different volumes of reproduction required, but also the setting of the condenser to correspond to these volumes, in order that the fiat characteristics shown in Fig. 4 may be preserved at thediflterent volume levels. It is preferable, however, to have a common adjusting handle for both the condenser 22 and the potentiometer 13, so that an attendant may, by a single manipulation, change from one volume to another, and at the same time automatically compensate for the volume-frequency distortion. This is indicated diagrammatically in Fig. 3 by the dotted connection 23. It will be understood thatigtpelpresentation of condenser 22 in Fig. 3 is in e to be schematic and that any form of variable condenser suitable for coupling low-frequency amplifiers may be employed. For example, this coupling capacity may take the form of any suitable variable capacity arran ement adapted to operate in unison with the. rush 16.

While in the foregoing descri tion it has been chosen to illustrate specific orms of ap paratus for accomplishing the automatic volume-frequency distortion correction, it is understood that other well-known. apparatus for accomplishing this result may be employed. For exam le, instead of coupling the tubes 15 and 20 y means of a variable condenser, a variable attentuating network comprising electrical impedance elements of the type disclosed in the patent to G. A. Campbell, No. 1,227,113, issued May 22, 1917, may be employed. Furthermore, the rising characteristics of amplifier gain represented by curves 6, c (Fig. 4) may be accomplished by so designing the inductance of transformer T and condenser 22 that resonance points may be formed at difierent frequency ranges.

Thus, as volume is increased, the tuning of condenser 22 may be such as to make the coupling circuit more resonant to the higher frequencies.

What I claim is:

1. The method of electrically transmitting and reproducing sounds which comprises translating the sounds into correspondingly modulated electrical currents, varying the gain of one electron discharge amplifier to produce varying volumes of reproduced sound, and simultaneously varying the gain of another amplifier selectively as regards lI I HGOOI'dMICG with the substantially flat cui've ded both volume and frequency to the extent required to maintain the aural efl'ects of the several components in the same proportionate ratio as in the original sounds.

2. In a sound transmitting and reproducing system, means for generatin sound modulated currents, a first electron ischarge device for amplifying said currents, means for varying the gain of said amplifier in accordance with the volume of reproduced sound desired, a second amplifier for further amplifying said currents, variable coupling means between said amplifiers, said coupling means having, for any one setting, radual- 1y changing transmission efiiciency or components of different frequency along the frequency scale of audible sounds and means for simultaneously varying said gain control device and said coupling means, said control device and said coupling means being coordinated to produce a diflferent predetermined non-linear volume-frequency relation at each gain adjustment.

3. In a system for electrically reproducing sounds with the same aural effects as the original sounds, means for translating the sounds into corresponding modulatedelectric currents, means for amplifying said currents comprising an electron discharge amplifier of a plurality of stages, means for coupling certain of said stages comprising a variable electrical network and means for varying the gain of one of said stages and for simultaneously varying said coupling network to compensate for volume-frequency distortion.

4. In a system for electrically reproducing sounds with the same aural effects as the original sounds, means for translating the sounds into corresponding modulated currents, means for amplifying said currents comprising an electron discharge amplifier having a lurality of stages, means comprising an adustably tuned circuit for coupling certain of said stages, and means for varying the tunin range of said coupling simultaneously Witfi the gain of another stage of said amplifier to compensate for volume-frequency distortion.

5. In an electrical sound wave amplifying system including a space discharge amplifier, means in the input of said amplifier for adjusting the gain to different levels, and means associated with the output of said amplifier having a non-linear gain-frequency characteristic discriminating in favor of higher frequency components of the audible range over the lower frequency components and means for actuating said first two means in unison to cause the discrimination in favor of the higher frequency components to be greater and greater as the gain level is increased through successive levels.

6. In an electrical sound wave amplifying system including a space discharge amplifier, means in the input of said amplifier for adjusting the gain, and adjustable means in the output circuit having for each adjustment a eater transmission efl'iciency for the higher equency components of the sound waves than for the low, said two means conjointly and simultaneously operatin to produce a different non-linear volumerequency relation in the amplified waves at each gain setting, corresponding to the response of the ear, whereby volume-frequency distortion is avoided.-

7. 'A space discharge amplifier system having an input circuit containing means for converting sound into electrical waves and an output circuit containing means for converting the amplified electrical waves into sound, adjustable means also in the input circuit for controlling the gain of said system, adjustable wave correcting means in the output circuit operative at each adjustment to mold the output components to the natural volume-frequency response characteristic of the ear, and unitary control means for said two adjustable means.

In testimony whereof, I have signed my name to this specification, this 17th day of May 1927.

J. LOUIS REYNOLDS. 

